Seoul (AFP) - A North Korean patrol boat repeatedly crossed the disputed Yellow Sea border with the South in an apparent show of force at the start of South Korea-US military drills, Seoul's defence ministry said Tuesday.The incursion took place three times overnight Monday and at one point the North Korean naval vessel had reached two nautical miles inside the South side of the border.

No shots were fired and the patrol boat eventually retreated after warnings from the South Korean navy, defence ministry spokesman Kim Min-Seok said."We suspect this is aimed at testing our military preparedness", Kim told reporters, saying it was apparent that the vessel had "intentionally violated" the boundary.

North Korean incursions over the maritime border -- which it does not officially recognise -- are not unusual and there were at least three last year.